1. Field of the Invention
This invention relates to an aesthetic orthodontic bracket having an archwire-engaging, load bearing framework embedded within a plastic body.
2. Description of the Related Art
Orthodontic treatment concerns movement of malpositioned teeth to orthodontically correct positions. In certain types of orthodontic treatment, tiny, slotted appliances known as brackets are affixed to anterior, bicuspid and cuspid teeth, and other appliances known as buccal tubes are secured to the molar teeth. An orthodontic archwire is placed in the slots of the brackets and is tied in place using small elastomeric O-rings or metallic ligature wires. Ends of the archwire are inserted into channels of the buccal tubes.
As orthodontic treatment progresses, the archwire serves as a track to guide movement of the teeth toward desired positions. Bends, loops or twists are often placed in the archwire to urge movement of the teeth. Alternatively, one or more elastic members secured to brackets or buccal tubes of certain teeth provide a force to move the associated teeth to their correct positions.
Many orthodontic brackets are made of metallic materials such as stainless steel. Metal brackets are often considered to provide satisfactory sliding mechanics, adhesion to teeth and debonding characteristics. However, metal brackets are visible in the oral cavity and often an embarrassment to the patient.
Ceramic brackets made of a color-free, translucent polycrystalline material provide an advantage over metal brackets in that the ceramic material permits the natural color of the tooth to diffusely show through the bracket. Examples of such aesthetic brackets are described in U.S. Pat. No. 4,954,080 which is assigned to the assignee of the present invention.
Orthodontic brackets made of plastic materials are also sometimes considered more aesthetic than metal brackets. Some plastic brackets are essentially color-free, and contain fiber or other filler material that scatters light transmitted through the bracket. Plastic brackets that scatter and diffuse light are often considered more aesthetic than transparent plastic brackets that may glisten and hence be more visible when in use in the oral cavity.
One problem long associated with plastic brackets, however, is the tendency for the plastic to deform or creep in areas surrounding the archwire slot. If the archwire slot widens, precise control over the position of the bracket and hence of the associated tooth is hindered.
Moreover, some plastic brackets are not as strong and resistant to fracture as their metal counterparts. Such plastic brackets may fracture in use, especially in narrow regions of the bracket between the archwire slot and ligature undercut regions located behind tiewings of the bracket. Replacement of the broken brackets is a nuisance to the orthodontist as well as to the patient, and can prolong treatment.
Attempts have been made to improve plastic brackets by use of a metal liner covering the archwire slot opening. An example of such construction is described in U.S. Pat. No. 4,299,569. The metal liner provides metal-to-metal contact between the archwire and the bracket so that the sliding mechanics of the bracket resemble sliding mechanics observed in connection with the use of metal brackets.
However, metal archwire slot liners such as described in U.S. Pat. No. 4,299,569 are not entirely satisfactory. For example, when such brackets are subjected to torquing forces as may occur when an attempt is made to pivot the long axis of the tooth in a direction either toward or away from the tongue, the archwire slot liner may deform along with deformation of the plastic. Once the liner spreads open a sufficient extent and no longer complementally engages sides of the archwire, precise control over movement of the associated tooth is significantly hindered.
A reinforcing framework for plastic brackets is described in U.S. Pat. No. 3,930,311. Such construction is an advantage over plastic brackets having a metal liner, in that the framework stiffens the bracket and is considered to distribute stresses over greater regions of the bracket than plastic brackets having archwire slot liners. In some of the embodiments set out in U.S. Pat. No. 3,930,311, edges of the framework are coplanar with edges of the archwire slot such that the archwire directly contacts the metal framework.
Presently, there is a need in the art for an orthodontic bracket having an embedded framework that efficiently and effectively distributes stresses throughout the plastic body from forces imposed by the archwire, and which also provides improved control of the bracket and the associated tooth during various types of orthodontic treatment. It would also be desirable to provide a plastic bracket having improved aesthetic characteristics in comparison to presently known plastic brackets.